1. Field of the Invention
This invention relates to a process for the production of expanded particles of a polypropylene resin.
2. Related Art
There is a known method of preparing expanded particles of a polypropylene resin, which includes the steps of providing expandable particles of a polypropylene resin containing a volatile blowing agent, dispersing the particles in an aqueous medium contained in a closed vessel, heating the dispersion to a temperature higher than the softening point of the resin while maintaining the pressure within the vessel at a pressure above the vapor pressure of the blowing agent, and discharging the dispersion from the vessel for subjecting same to an atmosphere maintained at a pressure lower than that within the vessel, thereby the particles discharged are expanded. Examples of the volatile blowing agent include propane, butane, pentane, trichlorofluoromethane and dichlorodifluoromethane.
Such a method, however, has been found to involve some problems due to the use of such a volatile blowing agent. Firstly, the temperature at which the expansion is performed should be limited to a specific, narrow range, since otherwise the expandable particles are swelled with the blowing agents which are good solvents for the polymeric material. Therefore, the expansion ratio which is dependent upon the expansion temperature is unavoidably limited within a narrow range. Secondly, some of these blowing agents are dangerous because of their toxicity and inflammability and require the replacement with air after completion of the expansion. Such a replacement requires the recovery of the blowing agents not only because of their expensiveness but also because of their tendency to destroy the ozone layer surrounding the earth.
The present invention has been made with a consideration of the above problems of the conventional expansion method.